The ramifications of climate change are long and vast, but with all this talk and attention the subject’s been gaining it’s a bit surprising that the mood of humans, and how this in turn affects the world, has been greatly overlooked. For instance, a new study, which has collected and statistically analyzed data from more than 60 studies from a wide array of fields (archaeology, climatology, economics etc.) spanning across hundreds of years, found a link between seemingly small changes in temperature and other climate change parameters and rise in human aggression. This relation holds true for both small and large scale violence, from rapes and local murders, to large scale conflicts and wars.

We’re used to hearing about climate change fallouts such as rise in sea level, increased heat waves, greenhouse gas effect so on and so forth, but now it seems you can add rise in violence to the list as well, and considering currently global warming projections, the world is expected to become an ever violent place. The researchers, led by  Solomon Hsiang, an economist at Princeton University in New Jersey, state however that they do not have a definite answer why this happens, though.

Marshall Burke, from the University of California, Berkeley, said: “This is a relationship we observe across time and across all major continents around the world. The relationship we find between these climate variables and conflict outcomes are often very large.”

The scientists first went through hundreds of papers from a wide array of fields which they found relevant, before they eventually settled for 60 studies on subjects related to climate, conflict, temperature, violence, crime, and more. The challenge was to align each study to a common ground, so that each individual findings might be correlated. They did this by implementing a common statistical framework, which can be seen sort of like exchanging various currencies (euros, dollars, british pounds) into a single one.

Mr Burke said: “We want to be careful, you don’t want to attribute any single event to climate in particular, but there are some really interesting results.”

Degrees of violence

What the researchers eventually found can only be classed as striking: even for minor variations from the traditional temperature or rainfall levels, substantial increase in frequency of violent events was witnessed on all levels, from local crime to war. Concrete examples given in the study include domestic violence in Australia, police violent interventions in Amsterdam, ethnic violence in South American and Europe and more.

Also the study was careful not to assess contemporary violence only. For comparison purposes, climate data spanning back many centuries was assessed. This helped paint a clearer and broader picture of how man had adapted, or didn’t for that matter, to climate change. For instance, political instability and warfare and linked to widespread and lasting droughts around A.D. 900 in lands near the Pacific, which eventually brought the demise of Maya empire. Decades of drought interspersed with intense monsoon rains collapsed the Khmer empire (modern day Cambodia) in the 14th century.

“Archaeologists can actually observe how [Khmer] engineers were trying to adapt,” Hsiang said. “They were trying to keep up with the climatic changes, but in the end, even though they were the most sophisticated water engineers in the region at the time, it still seemed too much.”

“A lot of the civilizations that were nailed by climatic shifts were the most advanced societies in their region or on the planet during their day, and they probably felt they could cope with anything,” he said.

“I think we should have some humility [and] recognize that people in the past were very innovative and they were trying to adapt to these changes as well.”

The researchers estimate that a 2C (3.6F) rise in global temperature could see personal crimes increase by about 15%, and group conflicts rise by more than 50% in some regions.

The root of the conflict? Hot temper

Other scientists show skepticism over the drawn conclusions, like Dr Halvard Buhaug, from the Peace Research Institute Oslo, Norway.

“I disagree with the sweeping conclusion (the authors) draw and believe that their strong statement about a general causal link between climate and conflict is unwarranted by the empirical analysis that they provide,” Buhaug said.

“I was surprised to see not a single reference to a real-world conflict that plausibly would not have occurred in the absence of observed climatic extremes. If the authors wish to claim a strong causal link, providing some form of case validation is critical.”

The study strengthens the idea that climate change and human aggression are linked, but it doesn’t however address why this occurs. One hypothesis, rather straightforward, is that increase in temperature or rainfall are unpleasant by nature, and thus make people cranky. Cranky enough bash each other’s skulls apparently. Then there’s the economic and social aspects of climate chance consequences.

“When individuals have very low income or the economy of the region collapses, that changes people’s incentives to take part in various activities,” study first author Hsiang said. And “one activity they could take part in is joining a militant group.”

Hsiang has faith that the exact mechanism that underlies the two might be uncovered in the future. The researchers liken their situation to that of doctors in the 1930s who knew that smoking and lung cancer were linked but had not yet uncovered the mechanism.

“It took decades, but people did eventually figure out what was going on, and that helped us design policies and institutions to help mitigate the harmful effects [of smoking],” Hsiang said.

Likening a biological mechanism to social-economic one, however, seems to me like apples and oranges though. Whatever’s the case, their findings published in the journal Science are truly fascinating.